The invention relates to a solvent process for preparing thermosetting compositions containing polymerizable chromenes derived from aromatic propargyl ethers. The polymerized products exhibit an unexpected substantial improvement in the flexural modulus and flexural strength, compared to other state of the art thermoset resins.
U.S. Pat. No. 4,226,800 to Picklesimer teaches the direct polymerization of propargyl ethers to polymers. It reports a process wherein a phenolic material is reacted with propargyl bromide in aqueous sodium hydroxide solution to afford propargyl ethers. The resultant ethers are then thermally polymerized without solvent directly to hard polymers.
IBM Technical Disclosure Bulletin, Vol. 27, No. 4B, pp. 25-29, (September, 1984), also describes the direct polymerization of polypropargyl ethers to polymers. Specifically, the bulletin describes that a mixture containing a 1:1 ratio by weight of aromatic propargyl ethers: ##STR1## can be polymerized by heating at 130.degree. C. for several minutes and then at 250.degree. C. for at least an hour. The process described to prepare the propargyl ethers utilizes propargyl bromide.
M. Harfenist and E. Thom, in The Influence Of Structure On The Rate Of Thermal Rearrangement Of Aryl Propargyl Ethers To The Chromenes, J. Org. Chem; Vol. 37, No. 6, p. 841 (1972), describe the cyclization of monofunctional propargyl ethers to monofunctional chromenes in yields of 36 to 76 percent, by heating in a solvent such as dimethylaniline, trimethylene glycol, or dichlorobenzene.
W. Anderson and E. LaVoie, in Thermal Cyclization Of Substituted Aryl Propargyl Ethers. The Scope And Regioselectivity Of The Reaction In The Synthesis Of Substituted 3-Chromenes, J. Org. Chem., Vol. 38, No. 22, p. 3832 (1973), describe the thermal cyclization of substituted aryl propargyl ethers. Specifically, they report the cyclization of simple 3-aryloxypropynes to the corresponding chromenes in approximately 60 percent yield, using the solvent diethylaniline at temperatures between 210.degree. C. and 215.degree. C.
K. Balasubramanian and B. Venugopalan, in Studies in Claisen Rearrangements: Claisen Rearrangement of Bispropargyl Ethers, Tetrahedron Letters, No. 29, p. 2707, (1973), report the formation of bischromenes derived from the bispropargyl ethers of 2,7-dihydroxynaphthalene and hydroquinone by heating the latter compounds in N,N-diethylaniline.
S. Powell and R. Adams, in A Comparison of the Activity of Certain Unsaturated Groups with the Activity of the Allyl Group in Certain Ethers, J. Chem. Soc., Vol. 42, p. 646 (1920), report that phenyl and p-bromophenyl propargyl ethers decompose upon heating with or without a solvent to tarry mixtures.
In addition to solvent systems for chromene preparation, catalytic processes are also known. Zsindely and Schmid, in Sigmatropische Umlagerungen von Aryl-propargylathern; Synthese von 1,5-Dimethyl-6-methylene-tricyclo [3,2,1,0.sup.2,.sup.7 ]-oct-3-en-8-on Derivaten, Helv. Chim. Acta, 51, 1510 (1968), disclose the rearrangement of aromatic propargyl ethers to chromenes at temperatures of about 200.degree. C. Koch-Pomeranz, Hansen, and Schmid, in Die Durch Silberionen Katalysierte Umlagerung von Propargyl-phenylather, Helv. Chim. Acta, 56, 2981 (1973) demonstrate that this rearrangement is catalyzed by silver tetrafluoroborate or silver trifluoroacetate provided at between 45 and 330 mole percent. According to the authors, such catalysts enable the reaction to be carried out at lower temperatures (20.degree. C. to 80.degree. C.) using a benzene or chloroform solvent.
Balasubramanian, Reddy and Nagarajan, in A Novel Mercuric Ion Catalyzed Reaction of 1,6-Diaryloxy-2,4-Hexadiynes to 4-4'-Bichromenes, Tetrahedron Letters, 50, 5003 (1973), report the mercuric oxide and concentrated sulfuric acid catalyzed rearrangement of bispropargyl ether: ##STR2## into the bischromene: ##STR3## They obtained the bischromene in 61 percent yield as crude, impure material which had to be purified by column chromatography.
The prior art poses several disadvantages. First, U.S. Pat. No. 4,226,800 and the IBM Technical Disclosure Bulletin describe highly exothermic direct polymerization processes, i.e., the heats of polymerization exceed 1100 Joules per gram. Such high heats of polymeriation lead to an undesirable inclusion of voids in the polymer and thermal stresses, which deleteriously affect the resultant polymer.
Second, known methods of preparing chromenes contemplate the complete conversion of propargyl ethers thereto. Harfenist and Thom suggest this attempted complete conversion leads to deleterious tar formation and a resultant loss of yield.
Third, the prior art fails to utilize preferred catalysts. The Koch-Pomeranz process utilizes silver salts, which are expensive and which may lead to the formation of undesirable byproducts, e.g., benzofurans. The Balasubramanian process utilizes mercuric salts, which are toxic and raise environmental concerns. U.S. Pat. No. 4,226,800 and the IBM Technical Bulletin utilize propargyl bromide, which is relatively expensive, inaccessible on a commercial scale, and shock sensitive, according to Fire Technology5, 100 (1969).
Those in the industry would find great advantage in the following: (1) a polymer with excellent physical properties produced through a reaction having a relatively low heat of polymerization; (2) reactions which enjoy high product yields without substantial tar formation; and (3) reactions which utilize preferred catalysts.